When making columnar grained castings the mold is positioned on a chill plate and then heated to a temperature above the melting point of the alloy before filling the mold with molten alloy. The chill plate is used to insure the start of solidification in columnar form at the surface of the chill plate and columnar grain growth is continued upwardly by a steep thermal gradient moving upwardly from the chill plate as, for example, by changing the temperature surrounding the mold or by withdrawing the mold downwardly from within the heating chamber for the mold.
High thermal stresses develop in the mold where it rests on the chill plate and routinely the mold is made thick enough and/or of a suitable ceramic to avoid cracking from such stresses. Such thickness of the mold wall affects the rate of cooling during solidification of the alloy thereby affecting the grain structure. The ceramic used may also have a reaction with the molten alloy, thereby affecting the chemistry of the cast article.